Traditional automatic transmission controls effect powershift gear changes in accordance with a preselected shift map, e.g., at specified ground speeds, even if the engine is at full speed and full throttle and regardless of the engine load and shift being performed. For example, U.S. Pat. No. 6,254,509 discloses a power shift transmission driven by an electronically-controlled engine and a control system that momentarily modifies a magnitude of an engine throttle command signal during a shift of the power shift transmission. The momentary modification occurs independent of the engine load and the shift being performed.
Gear changes according to this method are inefficient and result in excessive wear to driveline components and unacceptable shift quality. For example, a downshift of the transmission may require acceleration of one or more components of the transmission, for example, the countershaft, even though the overall power output is more greatly reduced across the transmission as a result of the downshift. As a result, the engine may lug during the downshift or, if the engine is already lugged back, the downshift may result in an engine stall.
Moreover, the transmission clutches are required to absorb the change in kinetic energy stored in the system and the energy generated by the engine during the gear changes. Additionally, such systems can exhibit excessive shift shock when the new gear is engaged due to the difference between the engine speed and the transmission speed in the new gear.
The subject invention is directed to improving shift quality by minimizing shift acceleration and delay while ensuring that the acceleration, delay, and engine response during shifting meet with operator expectations.